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Structural control on oil first migration within Triassic shale, Favignana Island, Italy

Parrino N.* , Todaro S. , Agosta F. , Di Stefano P. , Napoli G. , Pepe F. , Renda P. & Martire L. 3
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1 Dipartimento di Scienze della Terra e del Mare, Università di Palermo
2 Dipartimento di Scienze, Università della Basilicata
3 Dipartimento di Scienze della Terra, Università di Torino
* Corresponding email: nicolo.parrino@unipa.it
Keywords: Structural diagenesis, First migration, Fracture network.

Fluid migration in rocks characterized by low values of primary porosity and permeability can be affected
by fractures, whose formation is strongly influenced by the diagenetical history of the host rock. The aim of
this study is to define the role of fracture networks on oil first migration within outcropping Triassic carbonatic
shales exposed in the northernmost sector of Favignana Island, which can be considered as potential source
rock in offoshore western Sicily. The study outcrops are located along the limbs and the nucleous of a syncline
that formed during Miocene with a N50E trending fold axis. The ouctrops show cm- to m-thick beds of
yellowish clayey marls and marls, which are alternated to thick beds of grayish to blackish stromatolitic and
loferitic limestones. This study is based on the integration of geological, structural, sedimentological and
microstructural studies with chathodoluminescence and Scanning Electron Microscopy (SEM) analyses.
The goal is to constrain both relative timing of deformation and chemistry of the detected organic matter.
Preliminary field results highligt presence of three sets of high-angle to bedding and one sets of low-angle
to bedding opening mode fractures made up of joints and calcite filled, syntaxial veins. They formed during
different episodes, in which the pre-existing sets were sheared forming tensile fractures at their extensional
quadrants. Analysis of thin sections in transmitted light allowed recognition of several bed parallel catagenetic
micro-fractures. Genesis of this kind of structures was related to the increased oil volume due to organic
matter maturation. The recurrent evidences of oil in the central part of veins related to the earlier sets suggest
that these were the only fracture sets that were formed during oil migration. Moreover, SEM microanalysis
highlights the presence of S, Na, Cl and Sr inside the vein hosting the organic matter. Cathodoluminescence
measurement observations show different brightness degrees in both the analysed vein mineralizations and in
their host rocks; in particular, more than one set of veins shows different mineralization phases while other are
characterized by the same brightness degree of their hosting rocks. Data and interpretation carried out during
this ongoing work provide the first evidence of structurally driven oil first migration in the Aegadian Islands.

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